1. Field of the Invention
The present invention relates to a switching power supply device, which converts an input voltage to an output voltage by controlling a switching element to be turned on and off and supplies a load with an output power corresponding to the output voltage, and relates to a semiconductor device provided with such a switching power supply device.
2. Description of the Related Art
Conventionally, for the purpose of improving power efficiency by lowering a power consumption in a power supply device for use in general household devices such as home appliances a switching power supply device including a semiconductor device, which controls an output voltage by controlling a switching element such as a transistor to be turned on and off, have been widely utilized. In recent years, it has been strongly required to reduce the power consumption during a standby state for the devices such as the home appliances in order to prevent global warming.
In general, under a light load such as the standby state, energy loss of the switching power supply device is dominated by a switching loss due to a switching operation. There are well-known techniques for improving a power supply efficiency (a ratio of output power to input power) during the light load. One of these techniques is an intermittent oscillation control, which controls the switching element during the light load to repeat an oscillation interval when the switching element is turned on and off and a stop interval when the switching element stops turning on and off.
For example, when a load current supplied from the switching power supply device to the load becomes smaller as the load becomes lighter, the output voltage of the switching power supply device increases, thus a current flowing through the switching element is controlled to be smaller. Further, when the load current becomes small, under a predetermined condition, the switching element is controlled to perform an intermittent oscillation, which repeats the oscillation interval when the switching element is turned on and off and the stop interval when the switching element stops turning on and off. In this way, the intermittent oscillation control during the light load allows a number of times of switching to be reduced, and therefore, the supply efficiency during the light load can be improved.
In general, a cycle of intermittent oscillation, which repeats the oscillation interval and the stop interval, is controlled to be longer with an increasing output voltage due to a lightness of the load, and therefore, an intermittent oscillation frequency corresponding to the cycle of intermittent oscillation is lowered. Because of this, even in the case of controlling the switching element to be turned on and off at a frequency of 100 kHz in a normal operation other than the light load, the intermittent oscillation frequency possibly falls into an audible-frequency range below 20 kHz during the light load. In particular, when a variation of the load current becomes relatively smaller in standby or the like, the intermittent oscillation frequency is fixed to a constant frequency. However, when this constant frequency concentrates toward a particular frequency within the audible-frequency range, there could have a big problem of occurring a sound ringing (it is called a transformer sound or a magnetostrictive sound) from a transformer and a capacitor generally used in the switching power supply device.
As described above, the intermittent oscillation control during the light load is an effective means for improving the power supply efficiency during the light load. However, there is a drawback of occurring the transformer sound from the transformer or the like when the intermittent oscillation frequency falls within the audible-frequency range. In order to solve the problem of the transformer sound, it has been well-known that a peak current of the switching element upon controlling the intermittent oscillation is reduced. However, in this case, since the number of times of switching increases, the power supply efficiency is worse. Accordingly, there was a trade-off relationship between the problem of the transformer sound and the power supply efficiency. In addition, although there is such a well-known method that the transformer is adhered or impregnated, this method becomes more expensive. Accordingly, the intermittent oscillation control method for improving the above trade-off has been required.
On the other hand, a DC-DC converter according to Patent Literature 1 of Japanese Patent No. JP3391384 B can perform an intermittent oscillation control with a saw-tooth voltage, and the DC-DC converter can perform a soft-start for limiting the peak of the current flowing through the switching element upon starting the ON-interval of the switching element, and therefore, the DC-DC converter can control the transformer sound.
In addition, a switching power supply according to Patent Literature 2 of Japanese Patent Laid-open Publication No. JP2008-92793 A can determine the highest intermittent oscillation frequency upon controlling the intermittent oscillation based on the basic oscillation frequency for turning on and off the switching element and the number of times of switching at the time of restarting the switching operation upon controlling the intermittent oscillation. In particular, for example, the basic oscillation frequency in the normal operation is set to 30 kHz, and the number of times of switching upon restarting the switching operation when controlling the intermittent oscillation is set to at least four or more. In this case, when the forth-consecutive oscillation is performed at the time of restarting the switching operation upon controlling the intermittent oscillation, and thereafter, the oscillation is stopped for one cycle, or when the oscillation cycle has one set comprising of five cycles, the intermittent oscillation frequency has the maximum frequency. Accordingly, the upper limit of the intermittent oscillation frequency becomes 6 kHz. In this way, the transformer sound upon controlling the intermittent oscillation can be suppressed by determining the upper limit of the intermittent oscillation frequency.
However, since the DC-DC converter according to the Patent Literature 1 limits the peak value of the current flowing through the switching element at the time of restarting the oscillation upon controlling the intermittent oscillation, there is a problem of increasing the number of times of switching. In addition, since the intermittent oscillation frequency concentrates toward a predetermined frequency set within the control circuit, it is difficult to significantly improve the trade-off between the transformer sound upon controlling the intermittent oscillation and the power supply efficiency.
In addition, the switching power supply according to Patent Literature 2 can determine the minimum cycle of the intermittent oscillation cycles by controlling the number of times of switching within the period of restarting the switching operation upon controlling the intermittent oscillation based on the basic oscillation frequency of the switching element. In other word, the switching power supply according to Patent Literature 2 can reduce the transformer sound by setting the maximum of the intermittent oscillation frequency so that the switching element can not operate within the frequency area where the transformer sound becomes larger. However, the harmonic frequency of the intermittent oscillation frequency possibly falls into the audible-frequency area, and therefore, there is a problem of occurring the transformer sound. In that case, in consideration of the harmonic frequency, it is required that the upper limit of the intermittent oscillation frequency is set to be much smaller than the lower limit of the audible-frequency area. In addition, if the bandwidth of the intermittent oscillation frequency becomes wider, in viewing of a feedback control, the operation of the switching power supply is intended to become unstable. Therefore, it is undesirable to easily lower the upper limit of the intermittent oscillation frequency. Accordingly, the flexibility of setting the upper limit of the intermittent oscillation frequency is low, then it is difficult to apply to a broad range of the switching power supply.